1. Field of the Invention
The present invention relates to an excimer laser device and method in which the frequency of full gas exchange within the laser chamber is reduced, and more preferably full gas exchange is made unnecessary.
2. Description of the Related Art
Deterioration of Laser Gas and Countermeasures Against It
In excimer laser devices, a mixture of halogen gas, noble gas, and buffer gas is enclosed within the laser chamber for generating laser oscillations. As the excimer laser device is operated, the halogen gas enclosed in the laser chamber is consumed, and the reduction in halogen gas causes the output light energy of the excimer laser to reduce. In the following, explanations are provided as examples for a KrF excimer laser and an ArF excimer laser, which use fluorine gas as the halogen gas.
The main reason for the reduction in fluorine gas is parts within the laser chamber in contact with the fluorine gas reacting with the fluorine gas to form fluorides. Also, besides reduction in fluorine gas due to the generation of these fluorides, the output light energy is reduced by various impurities (oxygen and water) from the outside atmosphere passing the O-ring and becoming mixed with the laser gas.
Hence conventionally Japanese Patent No. 2701184 has disclosed technology to prevent the reduction of output light energy. The following is a summary of the method. As the laser device is operated, when the measured value of laser output light energy falls below a target value, the charging voltage value of a condenser is increased. The charging voltage value is gradually increased, and when it reaches a predetermined upper limit value, the laser chamber is replenished with a small amount of new gas. Then a part of the laser gas, corresponding to the amount of the pressure rise due to replenishment, is exhausted. Gas exchanging by replenishing with a small amount of gas in the laser chamber and then discharging part of the gas is called “partial gas exchange”. The timing of partial gas exchange can be based upon parameters other than the charging voltage value (for example, the Japanese Patent No. 2701184 refers to the number of laser pulse oscillations).
Replenishing with a small amount of new gas replenishes the reduction in fluorine gas due to generation of fluorides, and restores the original partial pressure of fluorine gas. Also, discharging part of the laser gas exhausts part of the fluorides and impurities that entered the laser chamber from outside. In addition, the total gas pressure in the laser chamber is restored to the original total gas pressure. In other words, by partial gas exchange the concentration of impurities in the laser chamber is reduced, and the laser output light energy is increased (restored) corresponding to the amount by which the concentration of impurities is reduced. Therefore, it is possible to continue operating the excimer laser device while maintaining the output light energy at the required level and the charging voltage value within the appropriate range.
Laser Gas Control Device
FIG. 1 shows the configuration of the device that controls the replenishment of laser gas as described above. Laser gas is enclosed within a laser chamber 1. An ArF excimer laser is shown in the figure, so the laser gas is a mixture of fluorine gas, Ar gas, and Ne gas. If the partial pressure ratio (molar concentration ratio:unit %) of fluorine gas, Ar gas, and Ne gas is a:b:c, the partial pressure ratio a:b:c is for example 0.1:3.5:96.4. Also, to stabilize operation of the laser about 10 ppm of Xe gas is also sometimes added.
The power supply 2 includes mainly a charger, a condenser, and a magnetic compression circuit. The power supply 2 provides a pulsed high voltage between exhaust electrodes, which are not shown in the drawings, in the laser chamber 1. The laser gas between the electrodes generates light due to electrical exhaust when dielectric breakdown voltage is reached, and laser oscillations start. In accordance with the laser oscillations, output laser light 6 is emitted from the laser chamber 1.
The various operation parameters of the laser (total gas pressure within the laser chamber 1, output light energy, oscillation frequency, etc.) are input to a controller 3, which controls the laser based upon these values. The controller 3 outputs the charging voltage value of the charger to the power supply 2 as the instruction value.
The total gas pressure in the laser chamber 1 is measured by a pressure monitor 4, and the output light energy is measured by an output monitor 5. Each measured value is transmitted to the controller 3.
A first gas supply source 7 and a second gas supply source 8 are cylinders filled with laser gas to be supplied to the laser chamber 1. The partial pressure ratio of the gas within the first gas supply source 7 is F2:Ar:Ne=n×a:b:c (n>1). The partial pressure ratio of the gas within the second gas supply source 8 is Ar:Ne=b:c. The gas supply sources 7, 8 and the laser chamber 1 are connected by gas piping 10, 12, and valves 9, 11 are provided on the piping 10, 12. Also, the laser chamber 1 and an exhaust pump 13 are connected by a gas piping 14, and a valve 15 is provided on the piping 14.
The controller 3 controls the opening and closing of valves 9, 11 to carry out partial gas exchange. In a partial gas exchange, first a prescribed amount of laser gas is supplied to the laser chamber 1 from the gas supply sources 7, 8. Then laser gas corresponding to the amount by which the gas pressure has risen due to the replenishment is exhausted from the laser chamber 1 using the exhaust pump 13.
Example of Control of Partial Gas Exchange
FIG. 2 shows an example of partial gas exchange control flow. First, output light energy target value Et, the optimum control charging voltage range Vmin to Vmax (minimum value to maximum value), the threshold value Np of the pulse oscillation number N (timing frequency of carrying out partial gas exchange), and the amount of gas exchanged in one partial gas exchange process ΔG are input (Step S1). The optimum control charging voltage is the voltage range for which the output light energy variation is small and laser operation is stable. The optimum control charging voltage range is determined from tests.
Laser oscillation is started using the initial charging voltage Vinit (>Vmin), and for each pulse the output light energy E, the charging voltage instruction value V, and the pulse oscillation number N is measured (Step S2). The pulse oscillation number N is reckoned from when the laser gas is totally exchanged or from when latest partial gas exchange is carried out. In other words, when the laser gas is exchanged, N is cleared and the count starts again from one.
Also, the output light energy measured value E is compared with the output light energy target value Et (Step S3). If E<Et, the charging voltage value used for the next pulse oscillation is increased so that the output light energy E approaches the target value Et. In other words, to the charging voltage in the previous pulse oscillation VN a prescribed voltage value ΔV=g×(Et−E) corresponding to the energy difference (Et−E) is added, and this value is used as the charging voltage VN+1 in the next pulse oscillation (Step S4). Here g is a proportional constant. If E≈Et, then VN+1=VN (Step S5). If E>Et the charging voltage value used in the next pulse oscillation is reduced so that the output light energy approaches the target value Et. In other words, to the charging voltage in the previous pulse oscillation VN a prescribed voltage value ΔV2=g×(E−Et) corresponding to the energy difference (E−Et) is subtracted, and this value is used as the charging voltage VN+1 in the next pulse oscillation (Step S6). Also, the charging voltage VN+1 is compared with the charging voltage upper limit value Vmax (Step S10), and if VN+1<Vmax the procedure returns to Step S2 and a similar process is repeated. When VN+1≧Vmax the excimer laser device stops operating (Step S11). Thereafter all the gas in the laser chamber is exchanged, for example.
In parallel with the output light energy control in Steps S3 to S6, Step S10 and Step S11, the measured pulse oscillation number N is compared with the threshold value Np (Step S7). If N<Np, the procedure returns to Step S2 and a similar control is repeated. If N=Np, the partial gas exchange control explained in FIG. 1 is carried out (Step S8, Step S9), the procedure returns to Step S2, and a similar control is repeated.
As the pulse oscillation number N increases the concentration of fluorine gas in the in the laser gas reduces. Also the concentration of impurities increases, so in order to maintain the output light energy target value Et, the necessary charging voltage is gradually increased. If partial gas exchange is not carried out the charging voltage instruction value V will reach the upper limit value Vmax, at which point a full exchange of the laser gas is carried out. In other words, partial gas exchange is a control that reduces the frequency of full exchange of laser gas by reducing the charging voltage instruction value V by restoring the partial pressure ratio of fluorine gas to the original value before the charging voltage instruction value V increases to reach the upper limit value Vmax, and by discharging part of the impurities in the laser chamber. Nonetheless the impurities are not completely exhausted, so eventually the charging voltage instruction value V will reach the upper limit value Vmax, and a full exchange of laser gas must be carried out.
Next the meaning of the mixing ratios of the gas in the first gas supply source 7 and the second gas supply source 8 is explained. The partial pressure ratio a:b:c within the laser chamber 1 is determined based upon the range for which the laser operates well. Good operation means the output light energy is high (oscillation efficiency is high), variation of output light energy is small, the oscillation spectrum width is narrow for narrow spectrum excimer lasers for semiconductor lithography, and so on.
The partial pressure ratio of the halogen gas, noble gas, and buffer gas in the mixed gas of the first gas supply source 7 is n×a:b:c (n>1). The partial pressure ratio of the noble gas and buffer gas in the mixed gas of the second gas supply source 8 is b:c. Therefore, by replenishing the laser chamber 1 with gas from the two gas supply sources 7, 8 in any proportion, the partial pressure ratio of noble gas and buffer gas in the laser chamber 1 is maintained at b:c. Also, the concentration of fluorine gas in the first gas supply source 7 is n×a (n>1), which is higher than the target value a for partial pressure ratio in the laser chamber 1. Therefore, by adjusting the gas supply ratio from the two gas supply sources 7, 8 it is possible to approach the target value of partial pressure ratio a for fluorine gas in the laser chamber 1. The supply ratio may be obtained by directly measuring the partial pressure ratio of fluorine and determining the deficit, or by determining in advance the correlation between fluorine gas reduction amount and pulse oscillation number N, and obtaining the supply ratio using this correlation.
In the partial gas exchange disclosed in the Japanese Patent No. 2701184, partial gas exchange can be carried out while the laser is operating. Therefore there is no necessity to stop the semiconductor lithography process, and by supplying new gas the frequency of full laser gas exchange can be reduced.
However, in the Japanese Patent No. 2701184 when the charging voltage instruction value V reaches the upper limit value Vmax, a full laser gas exchange is carried out. To fully exchange the laser gas, laser operation is stopped, so for example the semiconductor lithography process is stopped, which has the problem that production throughput is reduced.